NUTRITION 77 



pends the existence of a very large number of organisms 

 which break down these complex substances to simple 

 nitrates, the nitrogen compounds which alone are useful to 

 the majority of green plants. 



The organisms accomplishing these decompositions take 

 into their own bodies nitrogenous and non-nitrogenous car- 

 bon compounds elaborated by other and higher organisms, 

 reconstruct and assimilate some of the substance, making it 

 a living part of themselves, decomposing the rest by physio- 

 logical oxidation or by anaerobic respiration in order to 

 obtain energy. In the farmer's manure pile there are count- 

 less dependent organisms which fall into separate species, 

 easily conceivable but most difficult to isolate. On the sur- 

 face of the pile are fungi and bacteria which respire aerobi- 

 cally and attack mainly the non-nitrogenous matters, the 

 cellulose walls in the fragments of straw, and other vege- 

 table remains. Within the pile are the anaerobic organisms, 

 the iirst set living on the proteids and amides contained in 

 the animal and vegetable matters, building up their own 

 body substance from some of these and decomposing others. 

 Living upon the decomposition products and upon the dead 

 bodies of the first is the second set, which similarly build up 

 and break down. A third set subsists on the products and 

 upon the remains of the second; and so on down to the 

 nitrite and nitrate bacteria which oxidize ammonia (the 

 ultimate product of a great number of decompositions ) to 

 nitrites and these to nitrates respectively. 



When the highly complex protoplasmic substances of ani- 

 mals and plants, upon which few organisms can live, are 

 broken down to ammonia, water, carbon-dioxide, etc., and 

 the ammonia is oxidized to nitric acid, green plants can 

 begin again the constructive processes which end in the for- 

 mation of living protoplasm from inorganic nitrogen com- 

 pounds of the simplest sort. Thus we have the cycle of 

 nitrogen in the physiology of living organisms. 



A number of plants — some leading an apparently inde- 

 pendent existence — are forced, or at least find it advanta- 

 geous, to add to their supply of nitrogenous matters hj 

 taking into their bodies organic (that is, carbon) com- 



